1. Field of the Invention
The present invention relates to a power steering apparatus with fluid under pressure and particularly to a device for controlling fluid pressure supplied to a power steering apparatus in response to the speed of a vehicle.
2. Description of the Prior Art
It is widely known that after being made increased by a power steering apparatus having an elastic torsion bar, rotary torque from a steering wheel is transmitted to the vehicle wheels. For example, U.S. Pat. No. 3,433,127 discloses a power steering apparatus, in which a control valve controlled with fluid pressure is provided between an input shaft and an output shaft and the pressure fluid from a pump is selectively supplied in accordance with the relative displacement of the control valve to power-assist the rotation of the output shaft so as to make the steering wheel operate. The provision of such a power steering apparatus makes easier the steering of the vehicle. In the steering of the vehicle, however, it is desirable to obtain a larger power during running at low speed than at high speed. To this end, it has been also proposed that the torque generated in a power steering apparatus is be controlled in accordance with the speed of the vehicle.
It is disclosed, for example, in U.S. Pat. No. 4,034,825 that the torque generated in a power steering apparatus is controlled in accordance with the speed of the vehicle. In the power steering apparatus of this U.S. Patent, a torsion bar is positioned within a hollow input shaft connected with a steering wheel, and there is provided a control mechanism with a recess being formed in the outer circumference of the input shaft and a plurality of balls inserted in the recess. The plurality of balls are arranged such that they press against the input shaft with fluid pressure when they are supplied with pressure fluid. The fluid pressure is controlled by a pump which is adjusted in accordance with the speed of the vehicle so that the fluid pressure is increased to press the balls forcefully against the input shaft when the speed of the vehicle is high so to thereby weaken the action of the power steering apparatus. In this case, the fluid supplied to the balls is obtained by controlling the high pressure fluid from the pump, and there occurs a large pressure difference between the fluid pressure and the main pressure of the steering apparatus. Therefore, there is the disadvantage that precise fluid pressure control for the control mechanism and the steering apparatus is required and therefore the arrangement of the valve is complicated.